1. Field of the Invention
The invention relates to an improved method of and an apparatus for performing a fuel injection into an internal combustion engine.
2. Description of the Prior Art
For better comprehension of the description and the claims, several terms will be defined below: The fuel injection according to the invention can be performed under stroke control or pressure control. Within the scope of the invention, the term stroke-controlled fuel injection is understood to mean that the opening and closing of the injection opening is effected with the aid of a displaceable valve member on the basis of the hydraulic cooperation of the fuel pressures in a nozzle chamber and in a control chamber. A pressure reduction inside the control chamber causes a stroke of the valve member. Alternatively, the deflection of the valve member can be effected by a final control element (actuator). In a pressure-controlled fuel injection according to the invention, by means of the fuel pressure prevailing in the nozzle chamber of an injector the valve member is moved counter to the action of a closing force (spring), so that the injection opening is uncovered for an injection of the fuel out of the nozzle chamber into the cylinder. The pressure at which fuel emerges from the nozzle chamber into a cylinder of an internal combustion engine is called the injection pressure, while the term system pressure is understood to mean the pressure at which fuel is available or stored inside the fuel injection system. Fuel metering means furnishing a defined fuel quantity for injection. Leakage is understood to mean a quantity of fuel that occurs in operation of the fuel injection system (such as a guide leakage), is not used for injection, and is pumped back to the fuel tank. The pressure level of this leakage can have a standing pressure, and subsequently the fuel is pressure-relieved to the pressure level of the fuel tank.
A stroke-controlled injection has already been disclosed by German Patent Disclosure DE 196 19 523 A1. The attainable injection pressure is limited here by the pressure reservoir chamber (rail) and the high-pressure pump to approximately 1600 to 1800 bar.
To elevate the injection pressure, a pressure booster unit is possible, of the kind known for instance from U.S. Pat. No. 5,143,291 or U.S. Pat. No. 5,522,545. The disadvantage of these known pressure-boosted systems resides in a lack of flexibility of injection and poor quantity tolerance in the metering of small fuel quantities.
In a fuel injection system described in Japanese Patent Disclosure JP 08277762 A, two pressure reservoir chambers with different pressures are provided for the sake of increasing the flexibility of injection and the metering accuracy of the pre-injection. These two pressure reservoir chambers require major engineering effort and high production costs, and yet the maximum injection pressure is still limited by the fuel pump and the pressure reservoir chamber.
For better metering capability of the injection and execution of fast switching times, according to the invention, an improved method of and apparatus for pre-injection and post-injection at a lesser injection pressure than the main injection can be performed replicably. A high Injection pressure at a low pressure in the central pressure reservoir chamber can be achieved. The high-pressure generation in the fuel takes place directly in the region of the injection (metering), so that the efficiency is increased as a consequence of a smaller high-pressure volume. The use of motor oil to trigger the pressure booster unit in one embodiment assures increased safety and reliability in the performance of the method. In another embodiment, the injection pressure can be generated hydraulically, while the portion generated mechanically by means of a high-pressure pump is stored in the pressure reservoir chamber and is not used for the injection. Because of the low pressure, the load on the high-pressure pump is reduced, since this pump is not used to fill the pressure reservoir chamber, but only for the injection per se.